In recent years, studies to reduce the size of particles have progressed. In particular, intensive study has been conducted to reduce the particles into nanometer size (for example, in the range of 10 to 100 nm) which can hardly be realized by methods of pulverization, precipitation, and others. Further, attempts have been made not only to provide particles of a nanometer order, but also to provide them with excellent monodispersity (the term “monodispersity” employed in the present specification refers to the degree of uniformity of size of particle diameters dispersed).
Such nanometer-sized fine particles are distinguished from bulk particles (bigger in size) and from molecules and atoms (smaller in size). That is, the nanometer-size fine particles are categorized in a new field between them stated above in size. Thus, such nanoparticles are considered to show unexpected new properties over the conventional sized particles. It is also possible to stabilize the properties of nanoparticles if the monodispersity can be improved. Thus, nanoparticles having such potential are attracting attention in various fields, and they have been studied vigorously in a variety of fields such as biochemistry, new materials, electronic elements, light-emitting display devices, printing, and medicine.
In particular, organic nanoparticles made of an organic compound involve great potential as a functional material, because the organic compounds, per se, can be modified diversely. For example, polyimide has been utilized in various fields because of, for example, the following reasons: polyimide is a chemically and mechanically stable material owing to, for example, its heat resistance, solvent resistance, and mechanical characteristics, and is excellent in electrical insulating property. A material obtained by turning polyimide into fine particles has been used in a wide variety of new applications by virtue of the combination of the properties and shape of polyimide. For example, as a technical proposal, polyimide having a fine-particle shape is proposed for use as an additive in a powder toner for image formation (see, for example, JP-A-11-237760 (“JP-A” means unexamined published Japanese patent application)).
In addition, among the organic nanoparticles, organic pigments are used in such applications as painting, a printing ink, an electrophotographic toner, an inkjet ink, and a color filter, and thus the organic pigments are now important materials essential for our everyday life. Particularly, organic pigments are demanded in high-performance with practical importance including pigments for an inkjet ink and a color filter.
Dyes have been used as the colorants for inkjet inks, but pigments are employed recently for solving problems of the dyes in water resistance and light resistance. Images obtained by using a pigment ink have an advantage that they are superior in light resistance and water resistance to the images formed by using a dye-based ink. However, it is difficult to give fine particles having excellent monodispersity and having nanometer size, so that the pigment particles can hardly penetrate into the pores on paper surface. As a result, such an image has a problem that the adhesiveness thereof to paper is weaker.
Further, the increase in the number of pixels of a digital camera, there is increased need for reduction in thickness of the color filter for use in optical elements such as a CCD sensor and a display device. Organic pigments have been used in color filters, of which thickness depends significantly on the particle diameter of the organic pigment, and hence it is needed to produce fine particles in a nanometer size, with having stability in a monodispersed state.
As for production methods of organic particles, studies are made on, for example, a gas-phase method (a method of sublimating a sample under inert gas atmosphere and depositing particles on a substrate), a liquid-phase method (a reprecipitation method for obtaining fine particles by injecting a sample dissolved in a good solvent into a poor solvent of which the agitating condition and the temperature are controlled), and a laser-ablation method (a method of reducing the size of particles by laser-ablation of a sample dispersed in a solution with laser). There are also reports on preparation of monodispersed nanoparticles having a desired particle size by these methods.
Of those, the liquid-phase method has been attracting attention because it is a method of producing organic particles excellent in its simplicity and productivity (see JP-A-6-79168, JP-A-2004-91560, and others).
The crystalline form and the nature of the surface of each of organic particles produced by the liquid-phase method can be controlled by adjusting conditions under which the particles are precipitated in accordance with, for example, the kind of solvent, the rate of injection, and temperature. JP-A-2004-91560 describes an example in which the crystalline form of a quinacridone pigment is adjusted in accordance with a poor solvent kind.
With regard to an improvement in dispersibility of particles, an organic pigment has been conventionally dispersed on an industrial scale by using various dispersing machines (such as a roll mill, a ball mill, and an attritor). In this case, however, a particle in the pigment is reduced in size, with the result that the viscosity of the pigment dispersion may increase. The increase in viscosity makes it difficult to take the pigment dispersion out of a dispersing machine, makes it impossible to transfer the pigment dispersion through a pipeline, and, furthermore, causes gelling of the dispersion during storage so that the pigment dispersion cannot be used. A dispersing agent that aids the dispersion, or a polymer that stabilizes the dispersion has been added for solving them, but it cannot attain a sufficient effect (see, for example, Pigment dispersion technique-surface treatment and how to use dispersing agent, and evaluation for dispersibility-(TECHNICAL INFORMATION INSTITUTE CO., LTD 1999)).
In an organic pigments dispersion for a color filter, in order to improve the dispersibility, a polymer or a pigmentary dispersing agent capable of imparting both alkali developability and dispersion stability needed for the production of a color filter is added (see, for example, JP-A-2000-239554). However, such methods have not satisfied the demand yet because of, for example, the following reasons: such methods require a long period of time for dispersing, and involves an increase in viscosity of the dispersion.
In addition, an example in which dispersibility is improved by using pigment particles prepared by the liquid-phase method has been reported. JP-A-2004-43776 describes an example in which pigment particles in a water dispersed state is prepared by the liquid-phase method. However, this method is a method of providing pigment particles in an aqueous dispersed state, and the document describes nothing about a method of providing pigment particles in an organic solvent dispersed state.
JP-A-2004-123853 describes an example in which pigment particles in an organic solvent dispersed state is provided by using pigment particles prepared by the liquid-phase method. JP-A-2004-123853 describes a method of precipitating the pigment by dissolving a pigment in a basic compound and/or a basic solution and adding a liquid of a neutral compound and/or a liquid of an acidic compound, or a neutral liquid and/or an acidic liquid. However, organic pigment particles obtained by the method have large primary particle diameters, and then the method has not sufficiently satisfied a demand for a reduction in particle size.